The present invention relates to an objective lens apparatus used for an optical disk player.
An objective lens apparatus whose objective lens holder is attached to a peripheral portion of a cylindrical shaft pivotably and slidably in the thrust direction has theretofore been used as an objective lens apparatus for an optical disk player. Namely, an objective lens of the objective lens apparatus is fit slightly eccentrically in the axial core of the objective lens holder so that the objective lens holder may pivot for tracking adjustment and may slide in the thrust direction for focal adjustment. This type of objective lens apparatus comprises a first coil for focal adjustment wound around the peripheral portion of the objective lens holder and a square second coil for tracking adjustment attached to overlie the first coil. The shaft serves also as one yoke portion. The other yoke portion has a notch portion at the outer periphery of the objective lens holder. A linear motor for driving the objective lens holder in the thrust direction is constructed by the first coil and a pair of yokes.
A pivotable motor for driving the objective lens holder is constructed by the second coil and the pair of yokes. The return path of the second coil is constructed to face the notch portion for producing the pivot moment in the objective lens holder by the electromagnetic force applied to the forward path of the second coil. The optical path of a laser beam reflected on a disk through the objective lens passes the path distant from the axial core of the objective lens holder.
The above-noted objective lens apparatus is disadvantageous in that the laser beam reflected on a disk through the objective lens is modulated especially by driving the objective lens in the tracking direction. Namely, the amount of light of the laser beam passing through the objective lens is at a maximum when the optical axis of the laser beam coincides with the optical axis of the objective lens, while the amount of light reduces when the optical axis of the laser beam separates from the optical axis of the objective lens. Moreover, the separation of the optical axis of the laser beam from that of the objective lens is disadvantageous as it causes the movement of the laser beam reflected on a disk through the objective lens, the loss of the amount of light, and polarization of intensity of the light distribution. The conventional objective lens apparatus is unable to shift the optical axis of the laser beam in accordance with the shift of the objective lens.
Further, the diameter of the laser beam constantly incident on the objective lens should be large enough to cope with the shift of the objective lens, and thereby the loss of the quantity of light becomes extremely larger.
This invention eliminates the above-noted drawbacks.
The objective lens apparatus according to the present invention eliminates the optical modulation of the laser beam caused by rotation of the objective lens holder in the tracking direction and improves the S/N (signal/noise) ratio of a signal detected from the laser beam. Further, the loss of the quantity of light is reduced.